TITANIUM

Titanium has as atomic number, in the periodic table, 22 and is an element belonging to the fourth group.

Titanium Allotropy: 

Pure titanium at room temperature crystallizes into a compact hexagonal structure up to a temperature of 882°C. This type of titanium is called alpha titanium. 

Pure titanium is also called unalloyed titanium, precisely because it does not have any alloying elements. The alpha phase is predominant and crystallizes into a compact hexagonal structure. This occurs at room temperature as it is below 882°C. 

This compact hexagonal crystal structure of titanium, where the pink dots in the figure are the atoms, has titanium atoms at the vertices that are arranged in this way when the structure crystallizes.

Compared to other materials, which crystallize with the same type of structure, titanium, at the level of the basal planes presents a greater proximity. This means that there is a sort of interpenetration between these basal planes that hinders slippage. In practice the atoms are closer, they are an obstacle to each other and the planes slide in a more difficult way. In the end the sliding of the planes occurs on the faces of the prism and not on the hexagonal bases: this means that the yield strength of pure titanium is quite high (600MPa).  

This process takes place at temperatures below 882°C.

Rising above 882°C titanium changes its crystal structure: from compact hexagonal to body-centered cubic (beta titanium). 

It is defined as Beta-TRANSUS temperature the lowest temperature for which there is the presence of only beta phase. For pure titanium this temperature is 882°C. This means that if you heat a piece of titanium, which at room temperature is completely in the form of alpha titanium, so with a compact hexagonal crystal structure, at a high temperature that exceeds 882 ° C, the crystal structure changes and you will still have pure titanium but it will no longer be alpha titanium but beta titanium. 

This happens because the crystallographic arrangement of atoms changes.

State Diagram:

A state diagram is a graph in which are described how many and which phases of a system are present at the variation of pressure/temperature of the system itself. 

The diagrams we are talking about in this case are at P=constant and are valid in equilibrium conditions, therefore for very slow transformations. 

These state diagrams are used to: 

• 1. Understand, as temperature and composition change, which phases are present (for example understand when titanium goes from alpha to beta) 
• 2. Understand how much, if there should be more than one phase at the same time, what is the quantity and the curve of the different phases 
• 3. Understand the solubility of one component relative to another or in another 
• 4. Understand if a solid solution is formed or if two phases are formed that are diffuse and not miscible with each other 
• 5. Understand the melting temperature of these phases  
• 6. Determine the temperatures at which the material changes phase.

An example of a state diagram is shown in the image. In this diagram you have two vertical axes indicating temperature. At the bottom are pure element A and pure element B. In between are intermediate compounds that have a certain percentage of A and a certain percentage of B.  By varying the temperature the phases of the material will vary, for example if you take pure A it will rise until the melting temperature of A brings A itself to melting.

If a little bit of B is put in A then a solid alpha solution will be formed and at a certain point, while rising in temperature, it will pass through a zone where there are both a solid phase and a liquid phase and then it will melt. 

In the zone in between you will have a miscibility gap where you have two solid solutions mechanically mixing, so the material has two phases, one alpha and one beta. 

The further you move towards B the more you move into a solid solution rich in B which is alpha + beta. You will arrive in the graph at the level of the rightmost portion where you have pure B.

Consider a real diagram (e.g. lead-tin diagram), of a certain alloy with a certain composition, which has at least 40% tin by weight. If you start from the top the alloy at that point is liquid and as you go down in temperature, when you enter the area where you have a solid solution with a liquid, it means that when you move in this temperature range, you will have crystals growing as you go down in temperature. Below the horizontal line the material solidifies completely and you will have a mixture of two phases (an alpha phase called gray and a beta phase called tau).

Metal alloys are also biphasic and we can work to improve the properties of these biphasic alloys for example by going up and down in temperature to get these phases to have a certain shape and arrangement.

Titanium in the Human Body

Joint replacement implants, dental, and orthopedic applications contain titanium or cobalt-chrome alloys. 

The presence of a titanium oxide layer on the implant surface is considered crucial for maintaining osseointegration and preventing corrosion of the titanium surface (1).

Titanium plates are widely used in clinical settings because of their high bone affinity. and titanium fiber plate can better facilitate bone tissue repair than conventional titanium plate (2).

A series of complexes containing titanium, Ti, as a metal center has shown to possess a wide spectrum of antitumor properties. This series belongs to the non-platinum metal antitumor agents which has been developed mainly in the past 20 years. The bis(beta-diketonato)titanium(IV) and titanocene derivatives appear to offer a different alternative for cancer chemotherapy which do not follow the rationale and mechanism of action of the platinum complexes. The hydrolysis of these complexes in aqueous and pseudo aqueous solutions is discussed and the interaction studies of titanium complexes with biomolecules are also presented to unravel the mechanism of action at molecular levels (3).

This study finds that titanium is underestimated in biology and believes that there are molecular mechanisms that can maintain it in a highly stable and nontoxic form (4).

Titanium studies

• Molecular Formula:  Ti
• Molecular Weight: 47.867 g/mol
• CAS: 7440-32-6
• UNII D1JT611TNE
• EC Number: 231-142-3
• DSSTox Substance ID: DTXSID3047764
• MDL number  MFCD00011264
• PubChem Substance ID 329800079

References_____________________________________________________________________

(1) Ferguson A.B., Akahoshi Y., Laing P.G., Hodge E.S. Characteristics of trace ion release from embedded metal implants in the rabbit. J. Bone Jt. Surg. 1962;44:317–336. doi: 10.2106/00004623-196244020-00008

(2) Takizawa T, Nakayama N, Haniu H, Aoki K, Okamoto M, Nomura H, Tanaka M, Sobajima A, Yoshida K, Kamanaka T, Ajima K, Oishi A, Kuroda C, Ishida H, Okano S, Kobayashi S, Kato H, Saito N. Titanium Fiber Plates for Bone Tissue Repair. Adv Mater. 2018 Jan;30(4). doi: 10.1002/adma.201703608.

(3) Meléndez E. Titanium complexes in cancer treatment. Crit Rev Oncol Hematol. 2002 Jun;42(3):309-15. doi: 10.1016/s1040-8428(01)00224-4.

(4) Loza-Rosas SA, Saxena M, Delgado Y, Gaur K, Pandrala M, Tinoco AD. A ubiquitous metal, difficult to track: towards an understanding of the regulation of titanium(iv) in humans. Metallomics. 2017 Apr 19;9(4):346-356. doi: 10.1039/c6mt00223d.